Btu Calculator
Calculate BTU requirements for heating and cooling systems. Free, fast, accurate — no signup, mobile-friendly, works in
Calculation Type
Room Dimensions
Environmental Factors
BTU Basics
Calculation: Cooling Load Calculation
Required BTU
Calculated heat load
Recommended Capacity
With 10% safety margin
System Tonnage
AC/Heat pump size
Power Equivalent
Electrical equivalent
System Size
Recommended system category
Daily Cost
Est. energy cost
Monthly Cost
Est. energy cost
Annual Cost
Est. energy cost
How it works
A BTU calculator estimates the heating or cooling capacity a room needs. The British Thermal Unit measures heat energy; for air conditioning, a rough rule is about 20 BTU per hour per square foot of floor area, adjusted for sun, ceiling height, and occupancy.
Cooling capacity
BTU/hr ≈ area (ft²) × 20 (adjust for sun, ceilings, people)
- area
- room floor area in square feet
- 20
- base BTU per ft² (raise for sunny/tall rooms)
Worked example
- Room = 300 sq ft
- Average sun and ceiling height
- BTU = 300 × 20
≈ 6,000 BTU/hr of cooling needed.
Good to know
- Add ~10% for very sunny rooms and ~4,000 BTU for a kitchen; subtract ~10% for heavily shaded rooms.
- Oversizing an AC makes it short-cycle, cooling without removing humidity.
- Add ~600 BTU for each person who regularly occupies the room.
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Frequently Asked Questions
What is a BTU?
A British Thermal Unit is the heat needed to raise one pound of water by 1°F. Air conditioners and heaters are rated in BTUs per hour, which describes how much heat they can move or produce.
How many BTUs do I need per square foot?
A common cooling rule of thumb is roughly 20 BTU per square foot of living space, adjusted for ceiling height, insulation, sun exposure, and climate. A typical 300 sq ft room therefore needs around 6,000 BTU; kitchens commonly get about 4,000 BTU extra, and each additional regular occupant adds roughly 600 BTU.
What happens if my AC is oversized or undersized?
An oversized unit cools fast but short-cycles, leaving the air clammy because it never runs long enough to dehumidify, and the frequent starts wear it out sooner. An undersized unit runs constantly without reaching the set temperature. Correct sizing beats "bigger is better."
How many BTUs is a ton of cooling?
One ton of cooling equals 12,000 BTU per hour, so a 2-ton central air system delivers 24,000 BTU/hr. The term comes from the heat needed to melt one ton of ice in 24 hours.
Do sunny rooms or high ceilings change the requirement?
Yes — heavily sun-exposed rooms need roughly 10% more capacity and well-shaded rooms about 10% less. Ceilings above the standard 8 feet add volume to condition, so scale the estimate up proportionally.